Air compressor with axial and radialflow stages



March 14, 1961 P. G. SCHWAAR AIR COMPRESSOR WITH AXIAL AND RADIAL-FLOW STAGES Filed June 4. 1957 3 Sheets-Sheet 1 lNVENOR 4 4,5, w 4km March 14, 1961 P. G. SCHWAAR 2,974,857

AIR COMPRESSOR WITH AXIAL AND RADIAL-FLOW STAGES Filed June 4I 1957 5 Sheets-Sheet 2 INNER PERIPHE E Y V Vm =150 INVENTOR 42, M Mvmdzw ATT RMEYS March 14, 1961 P. G. SCHWAAR AIR COMPRESSOR WITH AXIAL AND RADIAL-FLOW STAGES 3 Sheets-Sheet 3 Filed June 4. 1957 OUTER PERIPHERY INVENTOR Q/wzd aw, M ywez AIRCOMPRESSOR WITH AXIAL RADIAL- FLOW STAGES Pierre Georges Schwaar, 'Dammarie-les-Lys, France, as-

signor to Societe Nationale dEtude et de Construction j de Moteurs dAviation, Paris, France, a French com- Filed June 4, 1351, s nrso. 663,485 7 I'I' Claims priority, June 11, 19:56. 1. 01. 230-119 l Radial-flow compressors have a' mass-flow per unit time which depends on their overall diameter. Despite the need, in some applications, of large mass-flows, it is not possible to increase without restriction the overall diameter, more particularly in the field of aeronautical machines, in which it is important to reduce as far as possible the maximum cross-sectional area and the weight. In order to overcome this drawback, that is to say to increase the mass-flow without increasing the overall diameter of the compressor, whilst retaining the same pressure ratio, attempts have been made to increase the external diameter of the suction area of the radial-flow moving wheel or centrifugal wheel.

However, if the absolute flow velocity at the intake of the centrifugal wheel is entirely axial, the speed relative to the external diameter of the section of admission will reach prohibitive values under these conditions, taking into account the deflection to be obtained in the axial inlet portion of the centrifugal wheel.

When it is desired to increase the external diameter of the axial section of admission it is necessary to produce a flow having positive pre-whirl at the external inlet diameter of the centrifugal wheel. But this involves a reduction in the compression ratio obtainable for the fluid streams located in the outer part of the fiow. If it is desired to maintain a purely axial absolute intake velocity at the internal diameter, and thus to obtain the maximum compression ratio for the corresponding fluid streams, it is necessary to provide an equalising compression work on the upstream side of the axial inlet portion of the centrifugal wheel.

This preparatory work will become all the more necessary as the external'diameter of the section of admission of the said axial inlet portion becomes greater.

In order to obtain a satisfactory specific rate of flow, the ratio of the preparatory work at the external periphery and of the preparatory work at the internal periphery will be of the order of 1.5 to 2.0. Such a radial variation of the compression ratio cannot be obtained in an ordinary axial flow stage.

However, if an axial flow stage of the transonic type is added on the upstream side of the centrifugal wheel of the radial flow compressor, theconditions of flow in the latter enable the required variation of the work of compression to be precisely obtained, together with a very high specific rate of flow.

In this axial flow transonic rotor, each blade works with large pressure ratios and high Mach numbers at the tip, and with smaller pressure ratios and Mach numbers at the root or hub.

By interposing between this axial flow transonic wheel and the radial flow wheel a stationary ring of suitably designed guide-blades, the required flow will be obtained at the intake portion of the radial-flow moving wheel for an increased external diameter of the section of admission.

Such a combination thus enables the mass-flow of the centrifugal compressor to be increased without increas- Patented Mar. I4, i961 ing its external diameter, and a machine can thereby be produced with a compression ratio in the neighbourhood .of 6. It is this combination which forms the subject of the present invention.

The description which follows below with reference to the accompanying drawings will make it quite clear ,howthe' invention may be carried out.

Fig. 1 showsdiagrammatically in axial half-section one embodiment of a'compre'ssor in accordance with the invention; c c

Fig. 2a shows diagrammatically a cross-section of a ;j' blade of a centrifugal; wheel of the conventional type,

the cross-section 'being made by a plane parallel to the axis of rotation at the inner periphery of the blading; thisfigure also showsv the corresponding triangle of speeds.

-. Fig.2b is similar to Fig. 2a, but corresponds to the outer periphery of the blading.

Figs. 3a, 3b, 4a, 4b, 5a and 5b relate to the case of a compressor in accordance with the invention .constructed inaccordance with Fig. 1, but without inlet guide-ring; those of the figures, the numbers of which have the index a correspond to the inner periphery or the root of the various blades, and those having the index b correspond to the outer periphery or the tip; with this notation, Figs. 3a and 3b relate to the transonic stage, Figs. 4a and 4b to the stationary intermediate guide-blades, and Figs. 5a and 5b to the centrifugal wheel.

The compressor in accordance with the invention which is shown in Fig. 1 comprises successively in the direction of the flow: a ring of stationary guide-blades 1 rigidly fixed to the two concentric parts 2 and 3 of the stator, a ring of moving axial flow blades 4 rigidly fixed to the rotor 5 with its axis at a--aand forming a transonic axial flow wheel (the blades of which are of difierent outlines at the root and tip thereof, said outlines being designed so that the compression ratio of the moving blades of this wheel is greater at the tip than at the root, for example the compression ratio may be 1.3 at the root of the blades and 1.8 at the tip), a ring of stationary guide-blades 6, and finally a ring of radial-flow moving blades 7, the axial inlet of which follows the guide-blades 6. The space comprised between the external casing 3 and the hub of the wheels 4 and 6 is convergent towards the centrifugal wheel.

In the cross-section of the centrifugal blade 7 shown, the area 7a cross-hatched in dotted lines is that which corresponds to the increase in the inlet diameter of the radial-flow wheel. The numbers marked vertically give by way of example various values of the diameter in centimeters and the numbers marked horizontally indicate the peripheral speeds corresponding to these diameters.

A comparison of Fig. 2 on the one hand and Figs. 3 to 5 on the other hand illustrates the operation of a compressor in accordance with the invention and the technical advantage which may be derived from it.

In these various figures, the following notations have been adopted: u for the angular velocities, v for the absolute velocities (V being the value of this velocity in the mean plane), w for the relative velocities and Mw for the Mach number corresponding to the relative velocity.

In Fig. 2, there has been considered the case of a centrifugal wheel of the conventional type having, for example, an external inlet diameter of 40 cm. for an internal diameter of 18.5 cm. The value of the relative intake speed at the external periphery of the blades corresponds to a Mach number of 0.9 for a peripheral speed u-- 260 meters per sec.

Figs. 3a and 3b show that while maintaining the same amassespeeds of rotation and the same internal intake diameter, in a compressor in accordance with the invention, the transonic stage 4 carries out a preparatory work of compression which" is igreater'atthe e'xternalradius than at as the internal periphery. In faet,-itis found froni' the triangleof speeds shown in Fig. 3a that at' the internal periphery, there is obtained a productuAV elqualfto' 14,400, and from the triangleofspeeds shown in Fig. 3b

that atfthe external periphery of thetransonic stage, this product increases to 31,0(l or slightly more 'than doll'ble. In the velocity diagramgtheindex l relates to th'e leading edge v and the index .2 to theztrailing. edge o'fthe blade.

Figs. 4a and 5a show that at the internal periphery the stationary 'guide-blades ti reetify the speed of the fluid axially :beforeuits entry intoJthecentrifugal wheel.

Figs. '4b and 5 b show thatat the-external periphery, the guide-blades 6 do not completely rectiiy the absolute speed of the fluid issuing from the transonic stage, 'so that the obliquity of this speed composed with the peripheral speed enables a Mach number to:be-'obtained-of the same order as that of Fig, 2b, 'but with an external inlet diameter of the wheel increased from 40 cm. to 45.4 cm., the periperal 'speed'being increased accordingly. In the velocity diagrams, the index 3 relates to the inlet eye of the radial-flow blade.

What is claimed is:

A multi-stage air compressor comprising, in succession along the air flow path, axial flow rotor blades, a stator ring of guide vanes, and a radial-flow stage with an axially directed intake end, saidaxial flow rotor blades having difiering aerofoil shapes at difierent -radial levels thereof,"th'e 'aerofoiIf-shapes at- 'th'e rodt and tip of said blades being respectively consistent with subsonic and supersonicirelative 110w conditions,- said blades having addition a spanwise 'twistconsistent with an increasing pressure ratio from*rootto tip,-said*'stator ringof guide vanes having diflering aerofoil shapes at different-"radial levels thereof, the trailinggedgeangle,ofsaidwanes being substantially zero at the inner portion'thereof and having a positive value at the outer portion thereof.-

References Cited in the file of this patent UNITED STATES-PATENTS 

